A population that belongs to the same species, but differs from other populations in terms of a certain trait as a result of having a different genetic composition, is referred to as a cultivar. In other words, even within the same species of plant, cultivation difficulty, resistance to damage caused by diseases and insects, yield, quality and the like differ according to the particular cultivar. Consequently, in agricultural products and particularly in major crops such as rice, barley or wheat, cultivar improvement has been carried out extensively in order to obtain better cultivars, and in recent years, cultivar improvement has been aggressively implemented by not only nursery companies and other private firms, but also by government agencies at the national and prefectural levels.
Accompanying recent progress made in fields such as nucleic acid analysis technology, the genes of various plants such as thale cress, rice and wheat have been analyzed, and the resulting genetic information has been disclosed. Cultivar improvement consisting of introducing a gene from an introduced species using genetic recombination methods is being carried out extensively by using this disclosed genetic information. However, although cultivar improvement by genetic recombination has the advantage of being able to introduce a trait possessed by a distantly related species for which crossbreeding is normally not possible, there is the problem of not always being able to adequately verify the safety thereof.
Consequently, new cultivars are being extensively produced by non-genetic recombination methods in the case of edible plants including rice. For example, Patent Document 1 discloses a method for producing a new cultivar having a target trait, without altering preferable traits possessed by the original cultivar, by controlling a substitution region using a chromosome fragment derived from an introduced cultivar in the case of substituting with an exogenous useful chromosome fragment by a non-genetic recombination method.
In rice in particular, cultivars are desired to be bred that mature a little earlier or a little later than conventional cultivars while still maintaining the same quality and yield of the conventional cultivar. Although the rice cultivar, Koshihikari, is cultivated in the majority of rice paddies of Japan since it is preferred by consumers, in the case of cultivating only Koshihikari on a large scale, harvesting-related work becomes concentrated in a short time period and requires considerable labor. Although there is no guarantee that each individual can be harvested at the optimum time particularly in the case of large-volume harvesting of rice, since harvesting the plants too early or too late has an effect on the flavor and yield of the rice, this presents a considerable problem for rice farmers. One possible method for shifting the harvesting time is to shift the seeding time. However, since Koshihikari is highly sensitive to light, even if the seeding time is shifted by only 2 or 3 days, it reaches the harvesting time at the same time. On the other hand, the harvesting time can be dispersed by shifting the seeding time by 10 days or more. However, in the case of making a considerable shift in the seeding time, the growing period becomes shorter, thereby resulting in the problem of being unable to obtain an adequate yield. If it were possible to cultivate a cultivar that matures a little earlier or a little later along with conventional cultivars, since this would make it possible to shift the harvesting time, harvesting work could be expected to be carried out while shifting the harvesting time for each cultivar.
However, the development of a cultivar that matures a little earlier or a little later than conventional cultivars, namely a Koshihikari cultivar that has been slightly adjusted so as to slightly shift the heading time or harvesting time thereof, is technically extremely difficult. This is because it is difficult to detect slight differences in heading time and specify a particular gene related thereto. In order to genetically detect such a slight difference in heading time, it is not sufficient to merely require a precise field at which soil and fertilization of the rice paddy, water, air flow and other conditions of the field environment are extremely uniform, but is also necessary for the status of the seeds used for seeding to also be uniform, and this is in fact extremely difficult. In actuality, in rice paddies in Japan, even among genetically identical cultivars, there is normally a deviation of about 7 days even if seeded on the same day and planted on the same day. In other words, there may be as many as 7 days between the day on which heading is first observed and the last day on which it is observed. Although fields of considerably high precision are used in the case of agricultural test fields and the like, there is still typically a shift of 3 to 5 days.